1. Field of the Invention
The present invention relates to a apparatus for forming a very uniform thin semiconductor layer on an insulating film provided on a base substrate in an SOI (Si On Insulator or Semiconductor On Insulator) structure which has a semiconductor layer on an insulator.
2. Description of the Related Art
Wafer bonding is one way of forming an SOI structure having a semiconductor active layer on an insulator. According to this method, at least one of two silicon substrates is oxidized to form a silicon oxide film which is an insulator, both substrates are put together with the silicon oxide film in between and are subjected to a heat treatment to be bonded together, and then the silicon substrate which will become an active layer is thinned down. Since the silicon substrate which is a single crystal is thinned down by this method, the SOI structure whose active layer has an excellent crystality can be obtained.
Conventional methods of thinning down the active layer include mechano-chemical polishing (MCP) and etching which uses an epitaxial layer as an etch-stop, as explained in detail in, for example, Journal of Electric Material, Vol. 21, No. 7, pp. 669-676, 1992). The former method has a difficulty in providing a uniform film thickness, and the active layer obtained by this method has a typical thickness and uniformity of 3 .mu.m.+-.0.5 .mu.m, and cannot form a thin film (whose thickness is 0.1 .mu.m, for example) which is effective in high-quality electronic devices. The latter method has a disadvantage of a complicated process for the epitaxial growth, which results in an increased cost. There is another method of precisely measuring the thickness of the active layer after mechano-chemical polishing and performs local plasma etching to etch the thick region longer than the other region, as described in, for example, 1992 IEEE International SOI Conference Proceedings, pp. 152-153. This method involves a complicated process and should face the plasma-oriented damage and contamination.
It is apparent from the above that while the bonded SOI has a great advantage of having an excellent crystality, it suffers a difficulty in providing an active layer with a uniform thickness, thus making it difficult to form a thin SOI structure effective in improving the performance of electronic devices. Further, the active layer in the bonded SOI structure was conventionally thinned down only by the method that suffers a lower precision in making the uniform thickness or the high-cost method that involves a complex process.